Abstract. To investigate the physico-chemical properties of aerosols in Taiwan, an
observation network was initiated in 2003. In this work, the measurements of
the mass concentration and carbonaceous composition of PM10 and PM2.5 are
presented. Analysis on the data collected in the first 5-years, from 2003 to
2007, showed that there was a very strong contrast in the aerosol
concentration and composition between the rural and the urban/suburban
stations. The five-year means of EC at the respective stations ranged from
0.9±0.04 to 4.2±0.1 μgC m−3. In rural areas, EC
accounted for 2–3% of PM10 and 3–5% of PM2.5 mass loadings,
comparing to 4–6% of PM10 and 4–8% of PM2.5 in the urban areas. It
was found that the spatial distribution of EC was consistent with CO and NOx
across the network stations, suggesting that the levels of EC over Taiwan
were dominated by local sources. The measured OC was split into POC and SOC
counterparts following the EC tracer method. Five-year means of POC ranged
from 1.8±0.1 to 9.7±0.2 μgC m−3 among the stations. It
was estimated that the POM contributed 5–17% of PM10 and 7–18% of
PM2.5 in Taiwan. On the other hand, the five-year means of SOC ranged from
1.5±0.1 to 3.8±.3 μgC m−3. The mass fractions of SOM
were estimated to be 9–19% in PM10 and 14–22% in PM2.5. The results
showed that the SOC did not exhibit significant urban-rural contrast as did
the POC and EC. A significant cross-station correlation between SOC and
total oxidant was observed, which means the spatial distribution of SOC in
Taiwan was dominated by the oxidant mixing ratio. Besides, correlation was
also found between SOC and particulate nitrate, implying that the precursors
of SOA were mainly from local anthropogenic sources. In addition to the
spatial distribution, the carbonaceous aerosols also exhibited distinct
seasonality. In northern Taiwan, the concentrations of all the three
carbonaceous components (EC, POC, and SOC) reached their respective minima
in the fall season. POC and EC increased drastically in winter and peaked in
spring, whereas the SOC was characterized by a bimodal pattern with the
maximal concentration in winter and a second mode in summertime. In southern
Taiwan, minimal levels of POC and EC occurred consistently in summer and the
maxima were observed in winter, whereas the SOC peaked in summer and
declined in wintertime. The discrepancies in the seasonality of carbonaceous
aerosols between northern and southern Taiwan were most likely caused by the
seasonal meteorological settings that dominated the dispersion of air
pollutants. Moreover, it was inferred that the Asian pollution outbreaks
could have shifted the seasonal maxima of air pollutants from winter to
spring in the northern Taiwan, and that the increases in biogenic SOA
precursors and the enhancement in SOA yield were responsible for the
elevated SOC concentrations in summer.